[llvm] [VPlan] Compute induction end values in VPlan. (PR #112145)

[Florian Hahn via llvm-commits]

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@@ -10078,6 +10053,56 @@ LoopVectorizePass::LoopVectorizePass(LoopVectorizeOptions Opts)
       VectorizeOnlyWhenForced(Opts.VectorizeOnlyWhenForced ||
                               !EnableLoopVectorization) {}
 
+/// Prepare \p MainPlan for vectorizing the main vector loop during epilogue
+/// vectorization. Remove ResumePhis from \p MainPlan for inductions that
+/// don't have a corresponding wide induction in \p EpiPlan.
+static void preparePlanForMainVectorLoop(VPlan &MainPlan, VPlan &EpiPlan) {
+  // Collect PHI nodes of widened phis in the VPlan for the epilogue. Those
+  // will need their resume-values computed in the main vector loop. Others
+  // can be removed from the main VPlan.
+  SmallPtrSet<PHINode *, 2> EpiWidenedPhis;
+  for (VPRecipeBase &R :
+       EpiPlan.getVectorLoopRegion()->getEntryBasicBlock()->phis()) {
+    if (isa<VPCanonicalIVPHIRecipe>(&R))
+      continue;
+    EpiWidenedPhis.insert(
+        cast<PHINode>(R.getVPSingleValue()->getUnderlyingValue()));
+  }
+  for (VPRecipeBase &R : *cast<VPIRBasicBlock>(MainPlan.getScalarHeader())) {
+    auto *VPIRInst = cast<VPIRInstruction>(&R);
+    auto *IRI = dyn_cast<PHINode>(&VPIRInst->getInstruction());
+    if (!IRI)
+      break;
+    if (EpiWidenedPhis.contains(IRI))
+      continue;
+    // There is no corresponding wide induction in the epilogue plan that would
+    // need a resume value. Set the operand in VPIRInst to zero, so ResumePhi
+    // can be removed. The resume values for the scalar loop will be created
+    // during execution of EpiPlan.
+    VPRecipeBase *ResumePhi = VPIRInst->getOperand(0)->getDefiningRecipe();
+    VPIRInst->setOperand(
+        0, MainPlan.getOrAddLiveIn(Constant::getNullValue(IRI->getType())));
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fhahn wrote:

At the moment it is needed, otherwise it would complicate VPIRInstruction::execute; but in the future we can disconnect the scalar header for the main vector loop, thus removing the need to update them there. (There is code elsewhere that use the scalar phi to get the resume value from the main vector loop)

https://github.com/llvm/llvm-project/pull/112145